1. Field of the Invention
The present invention relates to a signal separation apparatus including a longitudinally coupled resonator-type elastic wave filter apparatus that includes a plurality of longitudinally coupled resonator-type elastic wave filter devices having a two-stage cascade connection structure.
2. Description of the Related Art
A duplexer, which is a type of signal separation apparatus, used for mobile communication includes a transmission filter and a reception filter. To date, longitudinally coupled resonator-type elastic wave filter apparatuses have been widely used as reception filters. In Japanese Unexamined Patent Application Publication No. 2012-205215, a longitudinally coupled resonator-type elastic wave filter apparatus used as a reception filter includes first and second longitudinally coupled resonator-type elastic wave filter devices. In each of the first and second longitudinally coupled resonator-type elastic wave filter devices, two longitudinally coupled resonator-type elastic wave filter units are cascade-connected in two stages. In other words, the reception filter described above is a two-stage two-element longitudinally coupled resonator-type elastic wave filter apparatus.
In recent years, to realize a reduction in size, a duplexer, a type of signal separation apparatus, having a structure in which a transmission filter and a reception filter are formed on a single piezoelectric substrate has been used. With this structure, when the above-described two-stage two-element longitudinally coupled resonator-type elastic wave filter, which has a large longitudinal dimension, is used as the reception filter, a dead space is generated in the lateral direction. In addition, there has been a problem in that the degree of freedom in designing a transmission filter is decreased.
On the other hand, when a reception filter formed of a longitudinally coupled resonator-type elastic wave filter and a transmission filter are arranged in the lateral direction described above, characteristics are degraded due to mutual influence caused by leakage of signals.